Using the series rating charts – real example

Don’t mean to beat this topic to death. Well, yeah, I guess I do, but since I had another example of being over dutied/under rated come up today I thought I would share it.

Scenario: Fairly low impedance (2.9%) 300 kva dry transformer originally feeding a large dimmer rack. Over the years someone added a panelboard off the secondary of this transformer plus a cam-lock quick hookup for a mobile TV broadcast truck.

The branch panel is a Siemens S1 with a 125A type ED4 main breaker and a bunch of 1P20A type BL branch breakers. Voltage is 208Y/120V.

SKM model tells me I have a little over 10KA available at this panel. The BL breakers are only good for 10KAIC. Hmmm. What to do.

1. Replace the breakers with BLH’s or other 22KAIC rated breakers.
2. Artificially increase the feeder length until we get below 10KA available.
3. See if there is a tested series combination rated high enough.

See attached 3 page PDF for the solution. CLICK to get PDF and read more.